The cell surface hydrophobicity of strains of S. sanguis, S. salivarius, S. mitis and S. mutans was assessed by measuring: bacteria adsorption to polystyrene and glass surfaces; partitioning of 3H-labeled bacteria in hydrophobic-hydrophilic two phase systems consisting of hydrocarbon-water, polyethylene glycol-dextran, and ficoll-dextran; and adsorption of 3H-labeled bacteria to butyl-and octyl-agarose. The contribution of hydrophobic interactions to bacterial affinity for solid surfaces was demonstrated by increasing the salt concentration to 3M sodium chloride, which increased adsorption, and by adding potassium thiocyanate, a chaotropic agent, which decreased adsorption. S. sanguis cells showed the strongest and S. mutans cells the weakest tendencies to undergo hydrophobic bonding. Pretreating surfaces with human saliva tended to reduce strong adsorption caused by hydrophobic interactions. However some saliva coated S. mutans strains showed an increased tendence to undergo hydrophobic interactions. The potential of oral streptococci for hydrophobic interactions with solid surfaces in vitro thus appears significant. The contributions of such interactions in bacterial colonization of oral surfaces in vivo needs to be evaluated.